For many internal combustion engines, their engine control strategy has three important parameters: spark timing (or fuel injection timing in a diesel engine), the exhaust gas recirculation (EGR) rate and the air/fuel ratio (AFR).
To implement EGR, a fraction of the exhaust gas is recycled from the exhaust system back to the intake system. The recirculated exhaust gas is mixed with the fresh fuel-air mixture before entering the cylinders.
EGR can be implemented in internal and external configurations. External configurations can be implemented as either a high pressure or low pressure loop, depending on whether the recirculated exhaust is introduced to the intake post compressor (high pressure loop) or pre-compressor (low pressure loop).
EGR has a long history of use in both diesel and spark-ignited engines for reduction of NOx emissions. It affects combustion in several ways. The combustion is cooled by the presence of exhaust gas, that is, the recirculated exhaust gas absorbs heat. The reduction of peak gas temperatures in the combustion chamber reduces the production of NOx.
One approach to implementing EGR is with one or more dedicated EGR cylinders. In a “dedicated EGR” engine, the one or more dedicated cylinders are used to generate exhaust gas, all of which is dedicated to recirculation. With dedicated EGR, the quality of the recirculated exhaust can be improved with in-cylinder reforming of gasoline to hydrogen (H2) and carbon monoxide (CO). Subsequent combustion of this exhaust, mixed with the fresh air intake to produce the intake charge, is thereby enhanced.